Electrostatic process where the developer is the photosensitive medium

ABSTRACT

The method of producing xerographic images which comprises exposing a surface on which the image is to be produced to a light image while simultaneously directing a corona from an electrode remote of the said surface to the same surface so that the light image and corona field exist simultaneously on the surface and maintaining a film of liquid developer material on the surface during such exposure so that the toner medium receives a charge from the corona which charge is modified by the light where the toner medium is subjected to light so as to vary the electrical resistivity of the toner medium varies in light and dark areas and cause selective imagewise deposition.

United States Patent Brock et al.

ELECTROSTATIC PROCESS WHERE THE DEVELOPER IS THE PHOTOSENSITIVE MEDIUM Inventors: Alan J. Brock; John D. Brock, both of North Adelaide, Australia Appl No.: 332,396

Foreign Application Priority Data Feb. l8, l972 Australia 8003/72 US. Cl 96/] C; 9611.8; 96/].3 Int. Cl 603g 13/22 Field of Search 96/13, 1 PC, l C, l R

References Cited UNITED STATES PATENTS 2/]966 Hall 96/13 July 1, 1975 5 7 ABSTRACT The method of producing xerographic images which comprises exposing a surface on which the image is to be produced to a light image while simultaneously directing a corona from an electrode remote of the said surface to the same surface so that the light image and corona field exist simultaneously on the surface and maintaining a film of liquid developer material on the surface during such exposure so that the toner medium receives a charge from the corona which charge is modified by the light where the toner medium is subjected to light so as to vary the electrical resistivity of the toner medium varies in light and dark areas and cause selective imagewise deposition.

14 Claims, 3 Drawing Figures FATENTEEJM 1 SHEET mN\.. NM mm 3 ELECTROSTATIC PROCESS WHERE THE DEVELOPER IS THE PHOTOSENSITIVE MEDIUM This invention relates to a method of producing images using liquid or dry developers.

BACKGROUND OF INVENTION It is well known in the art of xerography to produce on a surface having a layer of photoconductive medium. a latent electrostatic image by charging the surface and then light modifying it and to then apply to the latent image bearing surface a developer which is selected to the image charge or the background.

Numerous inventions have been recorded in connection with xerography and the methods used vary greatly, some utilising dry powders applied to selenium surfaces, others utilising liquid developers applied to photoconductive surfaces, others using transfer methods whereby the images are produced on a photoconductive surface and then transferred to a nonphotoconductive medium, but it is the general practise excepting for some more recent approaches, in this type of image production to depend on a basic layer of photoconductive medium which it is possible to charge and to produce on it a latent image which then forms the basic membrane whereby the image can be rendered visible when a suitable marking medium is brought into contact with the surface.

Problems which exist with the commonly known systems are the inability to produce such images on plain paper on a non-photoconductive surface unless transfer from a photoconductive membrane is involved, but the act of transferring introduces certain complications such as spread of the image, the problem of achieving a correct transfer, the cost involved of using two membranes to produce an image, or the problems when using a rotating drum or the like of removing the previous image and producing the next image for transfer.

On the other hand when no transfer is involved it has been thought desirable that the medium on which the image is produced must contain the photoconductor if rapid and reasonably satisfactory images are required, and it is well known that one of the problems with treated paper, to give one instance, is the weight of the photoconductor on the paper which is objectionable for handling or mailing and which of course leaves the paper in a condition where it can be readily marked by contact with objects.

A further disability with any of the systems is that unless the fixing conditions are correct the image can be lost or can deteriorate with time.

According to a more recent system it has been proposed to use a pair of electrodes with a liquid developer there between in which one electrode is transparent and has the light image projected there through and both are conductive and have a field produced there between, the developer light sensitive and depositing according to this sensitivity but such a method is limited by the requirement that two electrodes must be used between which the action takes place with consequent drawing and other problems.

The object of the present invention is to produce a system which does not require a photoconductive surface to produce an image and does not require a field applied between a pair of electrodes through one of which the image must be projected.

While this at first may seem to be impossible, we have found according to our invention that it can in fact be achieved in a most satisfactory and highly novel manner.

SUMMARY OF INVENTION The basic consideration of the present invention is that the marking medium is itself the photoconductive medium which is energised in such a way that it deposits according to an electrical field and thus the deposition can take place on any plain surface of paper or any other suitable medium and moreover because the developer is the photosensitive medium and has the required light sensitivity, deposition takes place only on the areas required. the developer being deposited imagewise by using a corona which electrically charges the developer particles but light or other electromag netic waves discharge the developer particles so that they deposit pattern wise on ordinary paper or any other suitable surface under influence of the corona field.

The toner particles must be capable of taking a charge from a corona field to move in such field but also to have the charge modified in the areas of the image where light exists.

In its simplest form the developer has marking particles which have photoconductive properties and as an example the marking particles could comprise zinc oxide or a similar particulate photoconductor which is coloured to give the required marking medium, or it could be zinc oxide particulate material or the like which has been coated with a resin to form a compound particle, or it can be a photosensitive dye, but whatever the form, either the photoconductive medium or the resin or other associated medium is dyed or coloured or is selected according to colour so that it can go down in a selected area to form the actual image but will not go down in those areas where the deposition conditions do not exist.

Thus the basic principle of the invention is the use of a developer which itself has the necessary light sensitivity to activate the developer to go down imagewise.

To carry the invention into effect it will of course be necessary to use the differential effect of the photosensitive medium according to which the medium is conductive or insulating when light activated, which expression includes electromagnetic wave activation, and with this in mind the invention consists in the application of the developer to a surface on which the image is to be produced simultaneously with the projection on the surface of an image pattern or light or other electromagnetic waves so that developer present at this lo cality will vary in its charge holding properties according to the amount and colour of effective light striking the particular developer particles.

If now a field is applied at the same time as the image is projected onto the surface which is in contact with the photoconductive developer, it is possible to make the particles of the developer to move in accordance with the amount of light activation of the photosensitive developer at any particular site, and therefore the developer can be moved down in those areas where it is to be deposited but can be left in an inert state in those areas where no deposition is to take place, thereby producing an image which can then simply be fixed and is immediately in a finished condition.

So far as fixing of the image is concerned this can be carried out in various ways but again as an example the photoconductive medium in particulate form can be coated with a resin which is at least slightly soluble in the developer during the time of suspension in the developer carrier liquid so that when the image is developed and the particles moved down onto the surface in accordance with the selective action of the light and the field. these particles will adhere to the surface and when the image so produced is removed from the developer, and the partial solvent for the resin evaporates, the resin will be firmly bonded to the surface and a smudge free image will result.

In exactly the same way of course if a fixing medium is not necessarily associated with the photoconductive medium, it can be applied to the image before or after development has taken place, or a fixing medium which can be in solution in a carrier liquid can be applied overall irrespective of image selectivity by simply going down onto the surface on which the marking particles have been deposited, in which case what is equivalent to a glazed surface can be produced which however is still free of photosensitive media excepting in the actual image areas and therefore does not have the weight and other problems which result where a complete surface is given a coating of photoconductor or the like onto which the marking medium is then deposited.

As examples of how the invention can be carried out. dyed photoconductive particles such as zinc oxide can be suspended in a liquid such as a hydrocarbon having an electrical resistivity in excess ohm. cm. and a dielectric constant of for instance 3 and these developer particles can then be moved in the liquid to deposit on the surface where they are required in a differential manner according to the applied field and light modifying effect of the image.

It will be realised of course that it is not necessary to submerge the means on which the image is to be produced in a liquid developer because the paper or the like could move around a drum which has the surface of the paper or the like wetted just prior to formation of the image thereon, the wetting medium however being a developer ofthe character outlined, and that at an appropriate point the corona field and the light image will be applied to the wetted surface with the result that where the photoconductor or photoinsulator which is the developer is moved on to the surface, an image will be produced but at the other areas where the photoconductor is so modified that the field does not have an attracting force in the direction of the paper or other surface, the developer particles will not adhere to the surface and therefore can be readily removed by moving the paper past a clear insulating liquid or the like which will wash away the unfixed developer and which can contain a fixing medium. that is developer which is not held by the electrical field, but will leave the developer where it is moved and held by the existing field.

To enable the photoconductive medium which is now in a liquid having an insulating characteristic to discharge any inherent charge on it it may be desirable t somewhat lower the insulating value of the carrier liq uid, because it must be remembered that development according to this invention is a dynamic effect rather than utilizing static conditions. that is to say, there is no need according to this invention to first produce a la tent image which must be maintained while development is taking place after its formation, but rather in this invention the action is a dynamic one in that the field producing means such as a corona discharge and the modifying effect for the photosensitive medium are simultaneously present at the time when development takes place. and thus the carrier liquid can have a rela tively low electrical resistivity because its resistivity is not able to destroy the action due to both the field and the light modification being present at the same time.

In other words it is a continuous action and does not depend on maintenance of a previously formed latent image and the carrier liquid can be more conductive. say having a resistivity in excess of 10 ohm centimetre.

This is also one of the factors which makes the inven tion workable and which is one ofthe surprising results of the invention because wherein according to the older systems where the photoconductor required to bleed away the charge from an insulating surface to produce a latent image, in the present case the bleeding away of any charge on a photoconductor where the photoconductor is made conductive by the presence of light can take place through the carrier liquid in which the photoconductive particles are suspended. and the great importance of this will be realised when it is considered that this now opens the field to the use of a much greater range of carrier substances more suitable for the purpose, less toxic than many of the highly volatile insulating liquids used. before, and possibly also less expensive to manufacture or store or to ensure suspension of the particles in the liquid over longer periods due to the dynamic effect of the action. Thus more viscous liquis can also be used because relatively great forces can be built up on the photosensitive developer particles due to the maintained field and simultaneous light conditions as the image is produced. As there is only a base electrode and a corona. contamination is reduced.

It will be realised from the foregoing that an entirely novel and different method of producing images by what is akin to xerographic methods is involved, but the system should be classified as normal xerography because firstly it is a dynamic system in which the condi tions which move the particles remain active during any required time of deposition and factors such as image decay and the like are not present and also because of this dynamic effect, pointed out earlier in this specification, the liquids in which the development is carried out can actually be ofa relatively conductive nature to dissipate unwanted charges in areas where these are to be removed from the photoconductive developers to give the differential effect of the developers in the light and shade areas whereby selected deposition is then assured.

Obviously also another great advantage is that ordinary paper can for instance be used in such a process and the developer can serve to produce the image im mediately the light and field conditions exist and the only medium therefore required is in its simplest form plain paper on which an image is to be produced and a developer which can be immediately applied without precharging of a surface, without maintaining latent mages, and without any of the general complications .ech have been a feature of processes related to xerographic reproduction of images.

The invention can also be applied in dry developer processes provided the developer medium is suspended or moved in a medium such as a gas which can have the required characteristics to permit the light activated particles to discharge to attain the differential movement of charged and discharged particles.

In order, however, that the nature of the invention may be fully appreciated some embodiments thereof will now be described but it is to be clear that the invention need not necessarily be limited to these.

BRIEF DESCRIPTION OF DRAWINGS FIG. I shows somewhat schematically in side eievation a system in which a stack of sheets of paper are fed forward through a coating applicator station. the developer which is coated on at this station being then moditied by the projection thereon of an image and the application of a corona discharge, after which the excess developer is removed where not attached the sheet being then dried and the paper discharged to a receiving tray.

FIG. 2 shows a similar type of application but utilising a rotary drum feed, but again involving a stack of papers which are fed through a developer applicator stage and then imaged by the corona and light. after which the excess developer which is not forming the image is removed from the sheet and the sheet is dried and fed to a receiving tray.

In FIG. 3 is shown diagrammatically the general basis of the invention where (a) shows how the developer is applied to the paper and charged by a corona producing device and light modified at the same time, while (b) shows how the wash then removes the unattached developer material to leave only the image on the paper,

DESCRIPTION OF pREFERRED EMBODIMENTS Referring first to FIG. 1, a stack of papers 1 is carried on a platform 2 which is adapted to be fed upwardly by a lifting mechanism 3, such as a hydraulic mechanism. The stack of papers 1 is guided by the frame 4.

Immediately above the papers is shown a vacuum platten 5 which is connected through a line 6 to a suitable vacuum source. This is regulated so that when the vacuum platten 5 which moves on the guide 7 is energised. the perforate surface 8 of the vacuum platten engages the upper sheet of paper from the stack 1 and the platten is then moved in the direction of the arrow to cause the paper to traverse the developer applicator zone 9 which comprises a pair of rollers 10 oppositely driven to feed developer to the area 9 from a tank 11 containing the developer.

Developer 12 is fed to the tank 11 through a line 13 and surplus developer is drawn off through the line [4.

The developer comprising a particulate photoconductive or photoinsulative substance in an insulating medium. the photoconductive substance being tinted such as by including or having on it a resin coating which is adapted to hold a charge on its surface in that it is an insulator, but is slightly soluble in the carrier liq uid so that the surface of the particles is in a condition where when the surface to which it has been applied is thoroughly dried, the tackiness of the medium will cause permanent adhering to the surface.

The vacuum platten 5 passes on until it is at the imaging station 16, where an image is projected on to the paper by a projector 17 having a lens 18 and a transparency or the like 19, this then producing a light image on the photosensitive medium on the paper.

Simultaneously a pair of corona wires 20 are alternately actuated from a commutator 2] at a high frequency so that each wire is energised a number oftimes during the continuance of the illumination. the high voltage supply unit being designated 22.

Immediately such a surface has been subjected to the charge and light modified so that some ofthe photosensitive developer particles are now in a charged condition while other particles are in a discharged condition. the appropriate particles will be forced on to and will adhere firmly to the paper by the field. whereas the other particles will be only loosely associated therewith and not held thereon.

The vacuum platten 5 is now moved on to pass the paper over the washing station 25 which comprises a tank 26 having a supply of washing fluid. which may be similar to the carrier liquid, applied to it through the line 27 and removed through the line 28 so that there is a renewal of this liquid, or the liquid may be passed through filter or the like (not shown) by means of a pump, (also not shown) to maintain the required purity of the liquid. This liquid may be chosen to soften the developer particles for fixing purposes when the liquid is removed.

Within the tank 26 is a pair of rollers 29 which are driven in opposite directions by a motor or the like so that they carry liquid upwards on their outer peripheries to form a pool 31 of agitated liquid which is pro jected into the path of the paper being carried across this washing locality by the vacuum platten S. and the action of this is to remove any of the coating particles which are not rigidly held so that only the particles which were forced into firm contact by the field will re main in position on the surface of the paper sheet but the remaining developer particles will be removed and the final product will then be a plain sheet of paper similar to that which was fed into the machine from the paper stack excepting that an image is now produced on it by the adhesion at this stage of the coloured coating particles. It should be appreciated that when particles are forced into firm contact with a surface such as by a corona field such particles will be held both by the pressure contact established and by the slightly tacky nature of the developer particles when coated with a resin suspended in a carrier liquid in which the carrier liquid is slightly soluble.

Electrical field components also still exist on the particles and the paper or the like to increase the hold as no light should be permitted to strike the image after the corona and light are simultaneously switched off prior to moving the paper to the washing station.

As the platten 5 moves forward over a drying station 32, which can be supplied with a blast of heated or cold air supplied by a fan 33, any carrier liquid remaining, and washing fluid. is evaporated, thus causing the image to be firmly fixed to the paper surface due to the resin which is present and which hardens as the liquid is evaporated.

The platten 5 then moves on to the discharge locality 35 where the vacuum is terminated and if necessary a slight positive pressure applied to eject the sheet on to the tray 36.

The platten 5 then returns to its starting locality, and when a further copy is to be made. the vacuum is again applied and after engaging a paper the platten moves forwardly through the various stations to produce another copy.

Referring now to FIG. 2 where a rotary drum type of machine is shown, a paper stack 40, is guided by a frame 41 and rests on a lifting platform 42, which is urged upwardly by a hydraulic mechanism 43 or any other suitable loading such spring loading.

Situated immediately above the stack 40 is a drum 44 which has a plain periphery excepting at a paper picl-e up location where it has a perforate portion 45 behind which is a cavity defined by a wall 46 to form a vacuum platten 47. When suction is applied to the cavity the perforate portion 45 draws paper towards it and holds the sheet firmly on the perforate portion so long as the suction is applied, this being defined approximately by the line after which the suction in the platten is released and a positive pressure 1) applied over the are indicated in the illustration. the points marked corresponding in each case to the leading edge of the platten 47.

The action of the platten 47 is to draw the top sheet of the paper from the stack against the perforate portion and hold it there so that as the drum rotates in the direction of the arrow. a sheet of paper will be drawn out from stack 40 and will be firmly held on the platten 47 by the suction applied thereby.

Adjacent the paper stack 40 is the coating station 50 which comprises a tank 51 having within it a roller 52 driven by a belt 53 from the gear box 54 ofa motor 55, the tank 51 containing in it the developer 56.

The roller 52 is constantly driven in the direction of the arrow and that serves to keep coating medium agitated and also to keep the surface of the coating roller 52 wet with the coating liquid. Normally in contact with this coating roller 52 is an applicator roller 57 which has its surface wetted with the developer liquid 56, but the applicator roller 57 is under control of the solenoid 58 so that it can be pushed forwardly at an appropriate time to contact a sheet of paper from the stack 40 as it reaches this locality and thus apply coating material in the form of the developer to the surface of the paper.

When the applicator roller 57 is projected forward into contact with the paper it loses contact with the roller 52 and is then driven by contact with the paper, but as developer is carried upwardly by the roller 52 into the gap between the two rollers a constant supply of coating material is fed to the applicator roller 57 which is applied to the paper, any excess if such is present flowing back into the tank 51.

The light image is in this case produced from an original placed on to the curved transparent rest 60, and is scanned by a mirror 61, which moves at a rate such that scanning of the original takes place in proportion to the rotational speed of the drum 44, a lens 62 projecting the image from the mirror through a slit 63 on to the paper as it passes through this station.

The corona wires are indicated by 65 and are connected through the commutator 66 to the high voltage generator 67, the wires 65 being again alternately fed at a rapid rate for the purpose of obtaining a scanning sweep over the slit 63 as it will be realized that if both wires were fed simultaneously, the two Coronas produced would deflect to give a null zone on a medial level but by alternating the supply to the wires each in turn cover the slit 63 so that the result is a balanced charging action through the slit to the drum 44, the material forming the slit being insulating if necessary.

In this way, the surface of the paper has a developer image produced on it as it passes the slit 63, but imme diately it leaves this area it reaches the washing station 70 which comprises a wall 71 which may be an electrode. this wall being provided with three ducts 72, 73. and 74, the duct 72 being a central duct, and like the ducts 73 and 74 extends across the complete width of the drum the central duct 72 being a supply duct for the washing fluid, while the ducts 73 and 74 on either side of it are suction ducts which remove the fluid. the three ducts leading away to a mechanism which is not shown but which simply comprises a pump and means to apply a relatively strong suction to the ducts 73 and 74 so that cleaning liquid can be fed into the duct 72 and will pass in either direction from its outlet into the gap between the wall 71 and the paper on the drum 44 so that there is a turbulent washing area in this location, the liquid used to supply the duct 72 and which is removed through the ducts 73 and 74 being cleaned or purified as requiring during circulation and if desired the recov ered developer particles can be fed back to the coating medium.

Thus there is again a washing of the surface which contains the image and again the surface is cleansed of any particles of coating material which are not firmly held in position by the remaining latent charges and other factors referred to in connection with HO. 1 so that again a sheet of paper emerges from this zone which bears only the image and has any coating mate rial removed from the non-image areas.

From the washing locality the paper enters a drying station 76 and after leaving this is discharged from the platten by termination of the suction and by the application of positive pressure if required, to be received on the tray 77.

The drum can continue to rotate until a required number of copies has been produced because each time it reaches a position shown in FIGv 2 and the suction is applied, a sheet of paper will be picked up by the platten and will be moved first through the coating station then through the exposure station and then through the washing and drying stations to produce the final copy.

In FIG. 3 is shown somewhat schematically how the process operates.

At (a) is shown how the sheet 80 is placed on to a base 81 which is earthed and forms a base electrode. 82 represents a pair of corona discharge wires and these are connected to the high voltage generator 83 through a commutator 84. The high voltage generator is also earthed.

Excitation of the generator 83 causes a corona to be produced between the discharge wires 82 and the base electrode 8].

85 represents a flow of developer which forms the coating material into the light image and corona zone, this comprising photosensitive particles 86 held in a carrier liquid 87, this preferably being a liquid having an insulating value above l0 ohm centimeter photoconductor particles preferably being coating with a colouring medium corresponding to the colour of the final reproduction required, which colouring medium can be a resin slightly soluble in the carrier liquid, but more preferably in the washing fluid, so that when subsequently the liquid is dried off, any particles adhering to the sheet 80 will be bonded thereto.

It will be noted how assuming the developer to contain zinc oxide particles, uncharged particles, which enter the charge and light zone remain relatively uncharged (white circles) and do not move in the corona field. but when they enter the dark zone of an image. they become charged (black circles) and move in the corona field to deposit as shown at 89.

The projector 90 produces the white zones 9] representing a light image. the shaded zone 92 indicating dark areas.

It is obvious then that particles which contain a substantial charge will be held firmly in position while particles where the charge has been removed will be free to leave the sheet 80 when the surface tension and other hold is broken, and this is demonstrated in (b) where the washing liquid 93 sweeps the surface as defined for instance in the two embodiments described earlier as FIGS. 1 and 2, and it will be noted that the washing liquid which enters in the direction of the arrow 94. leaves in the direction of the arrow 95, carrying with it the photosensitive particles which do not have a firm attachment to the paper because they do not have a strong attraction, whereas the particles which were driven down by the corona have remained in position on the sheet 80 and, therefore. immediately the washing is completed and the sheet 80 is removed and allowed to dry, the particles which adhered are fixed to solvent action on the surface of the particles as previously mentioned, or they could be heat fixed or fixed in any other way.

The whole of the process of course must be carried out in the dark to maintain the image differential charge and the shaded line 96 represents that the stages illustrated must be carried out in the dark so that the only discharging which can take place is at the time of exposure to produce the image.

It is preferred to carry the removed photoconductor medium away by washing rather than to use a bias plate to lift it from the surface, because in this way the removal area is kept clean in that the removed particles are carried away and can readily be recovered at a remote area with saving of material and maintaining of clean washing fluid.

In FIG. 3 the drying step has not been shown but this can be carried out in any convenient manner.

Examples will now be described but in connection with these a general observation may help in understanding the principles of the invention.

When any particulate substance is subjected to a field in an insulating liquid its behaviour is governed by the insulating or conductive properties of the substance. Substances which are insulators tend to be held on an electrode longer than substances which are conductors but all substances oscillate in the field. The present invention takes advantage of this difference in movement and retention between insulating and conductive substances and uses as a developer any substance the conductivity of which changes in the presence of light.

Thus. photoconductor and photo-insulators may be used. the only requirement being that there shall be a difference in conductivity of those particles which are in the light part of an image and those which are in the dark part thereof. which positioning affects their conductivity and. thus. their mobility and retention on the surface which forms one of the electrodes and which supports the medium on which the image is to be produced.

It will be realised that the marking particles which. as said. must be light sensitive so that their conductivity changes when light is projected on to them and this. of course. means that if a selection is made of particles which are sensitive to one particular colour of light then those particles only will be activated when that light strikes them and. therefore. if three groups of marking particles are used each sensitive to one of the colours of a colour system. the correct particles can he made to go down in correct localities in accordance with a colour image which is projected on to the surface.

This is possible because the particles have their conductivity changed by a particular colour at the time that they are deposited on to the image and it will. therefore. be realised that multi-colour images can be produced during the one exposure by having the pri mary or complementary colour sensitive developer particles separately suspended in the developer liquid so that each colour when activated can develop the area where the required colour is required.

The following examples indicate a range of substances and their general behaviour.

EXAMPLE 1 A photosensitive suspension was prepared using the following materials:

Zinc oxide. Special Z. product of Durham Chemicals Aust. 5 grams lsopar G. isoparaftinic solvent of Esso Corp 200 grams By kneading these materials in a ball mill for about 3 hours a white toner suspension was produced which was fed into the exposure unit. During exposure and in the presence of an electric field the white zinc oxide particles migrated to the dark areas of the image producing a negative reproduction.

EXAMPLE 2 The photosensitive response of the above suspension in Example 1 was modified by introducing a polarity regulating agent such as Beckosol 1535 a product of Reichold Corp. of Japan.

EXAMPLE 3 The suspension of Example I was modified by introducing a soluble fixative into the Isopar G such as a hydrocarbon resin or a rubbery polymer such as Solprene 1206, a product of the Phillips Imperial Chemical Ltd.

EXAMPLE 4 The photoresponse of the zinc oxide in Example l. was modified by dyeing the zinc oxide with quinoline yellow. rose bengal. or bromophenol blue from 0.5 per cent methyl cellusolve solutions.

EXAMPLE 5 The zinc oxide was dyed with acridine yellow. methylene blue. bromophenol blue or naphthol green from l per cent methanol solution.

EXAMPLE 6 A photosensitive suspension was prepared using the following materials:

Watchung Red. product Dainichi the Danichi Seika K.K. of Japan .4 grams Isopar H, isoparaffinic solvent of Esso Corp. By kneading these materials in a ball mill for about hours a red toner suspension was produced which was then fed into the exposure unit During exposure in the presence of an electric field the red particles migrated to the dark areas producing a positive reproduction.

EXAMPLE 7 In Example 6. the Watchung Red was replaced with Benzidine Yellow GGR a product of the Toyo Ink. Co. oflapan or Phthalocyanine green LX or copper phthalocyanine Cl 74260 from Imperial Colour and Chemical Co. of USA or Irgalite GLSM phthalocyanine blue of Geigy Co. Switzerland.

EXAMPLE 8 In Example I the zinc oxide was replaced by cadmium selenide, X4401. product of Dry Colour and Chemical Manufacturing Co. Victoria, Australia.

EXAMPLE 9 In Example I the zinc oxide was replaced by cad mium sulphide, copper activated.

EXAMPLE 10 In Examples 1 and 6 the photosensitive medium was replaced by Naphthol Red B. or Calcium Lithol Red. or lndofast double scarlet toner. or Quindo Magenta, or Vulcan Fast Red BBE toner or the like.

EXAMPLE I 1 Colour Reproduction In the case of where colour reproduction is required such as by repetitive processing of a surface with different colour exposures and development an indication of the substances useful for this purpose either as dyes which are themselves light sensitive or as colouring medium for photoconductors or photo-insulators can be used and the following lists show materials whereby three basic colours can be produced, if necessary. by toning a particulate photoconductive medium.

REDS

ORGANIC Para Red Pigment Red l l2(l7(l (p-nitroaniline Z-naphthol] eg. Para Toners ex Smith Reichold (A.C. Hatrick Chemicals Aust.) Chlorinated Para Pigment Red 4 Red t2-chloro-4-nitroaniline Z-naphthol) cg Monolite Fast Red CIA ex I.C.l A.N.Z. Toluidinc Red Pigment Red 3 IZ-IBIRVP-HIIUILIIHC a Z-naplitholl cg Gruphtol Red A-J-RL ex Sandoz. Bon Arylamidc Pigment Red '9 Red CI. IZIZII 45150 lpliosphotangstomolylidic acid lakes with Xanthanesl eg. Fanal Pink 8 Supra ex BASF. Ausl.

-Continued INORGANIC Cadmium Reds Pigment Red I08 Cl WW6 (Cadmium sulphs selenidcs) eg. Cadmium Red c\. I.C.I.A.N 7.v Red Oxide Pigment Red l0lillll (.l. 7749] (Ferric Oxide) cg. Natural Spanish Red ()xide ex Hardic Trading BLLES ORGANIC hthtiloeyanine Pigment Bliic I5 ('I 'NIhll Blue teopperl e g. NIURLISIILII Fast Blue BGA ex l.(' IAN Z. Basic Dye Pigment Blue l.l..'l.4tfi.h.tt l-l Complexes (PhosphornolyholiclPhosphotungstonioh Iidtc acid lakes of basic dyes) eg Irgalite Blue F.R. ex Ciha-Geigy Reflex Blue Pigment Blue [8.56.67 Cl. 4277llA [Monosulphonic acid of phenylated lpara) rosanilincl e.g. Reflex Blue (16 ex Aust. Hoechst. Monoazo Blue Pigment Blue 23 CI cg Hclio Fast Brilliant Blue RR ex Bayer Lmerkusen lndanthroiie Blue Pigment Blue C l Nmtlll cg Monolite Fast Blue SR5 ex LC l A.l\'.7..

INORGANIC Ultramarine Blue Pigment Blue 29 CI. 77011? (Polysulphidc of Sodium (a K Li or Agi aluniino silicate) cg. L'ltraniarine Blue ex l.(.l..-\ N]. Iron Blues Pigment Blue 27 (.l. T'ISIU 77521) t Potassium (or sodium or ammonium l l'crric l errocyaiiidcl eg Prussian Blue ex Rohi. Bryce,

YELLOWS ORGANIC Arylamide Yellow Pigment Yellow 1 Cl. l 1680 (2I'lIIlOp't0IUILII11C aectoacettimlide) e.g. Monolite Fast Yellow GNA ex I.C.I.A.N.Z. Benzidine Yellow Pigment Yellow 12 (II. 21090 Anilide (3.3 dichloro henzidine acetoacetanilide) eg. Graphtol Yellow A-HG ex SLIIRIUL Mono Azo Yellow Pigment Yellow 97 Cl cg. Permanent Yellow FGL ex Austv Hoechst Benzidine Yellow Pigment Yellow l4 C I. 21095 Toluididc (l3'dichlorobeiizidine o ueetoacetotoluidide) cg. lrgalite Yellow BRE ex Ciha a Cieigy Monoazo Lake Pigment Yellov. 61 Cl. e.g. Irgalite Yellow W.S.C. ex (Tiha-( eihy Monoazo lake Pigment Yellow CI.

(Barium) eg. Graphtol Yellow 3 GLN ex SUI'ILIUL Flaxanthronc Vat Yellow 1 CI 706W) Yellow eg Monolite Fast Yellow FRS ex [.CIANZ lsoindolinone Pigment Yellow I It) Yellow Tctrachloro) e g. lrgazin Ycllovt JRLT ex Cilia (ieigy INORGANIC Cadmium Yellow Pigment Yellow 37 l. 77W) (Cadmitiiii Sulphide) cg. Cadmium Yellow ex. l.C I.A.N.7. Zinc Chromziic Pigment Yellow 36 CI. 77955 4 basic zinc potassium chroniiitel cg. Zinc Chromate e\ Aiist. Hoechst.

ZN 44A Yellow Oxide Pigment Yellow 42/43 CI 77492 lHydratctl ferric oxide) c g Yellow Iron ()\ide ex I.C.l AN].

We claim:

I. The method ol" producing images on a surface which comprises:

a. wetting and maintaining a film of photoconductive developer material on said surface. which developer comprises a carrier liquid with an electrical resistivity in excess of ohm centimeter and a toner medium which can receive a charge from a corona but which charge is modified by light when and where the said toner medium is subjected to light whereby the electrical resistivity of the said toner medium varies in light and dark areas and thus varies in retention delay characteristics on the said surface when in a field.

b. exposing the said surface with the wet film of photoconductive toner thereon to a light image.

c. simultaneously directing a corona from an electrode remote of the said surface to the said surface to extend uniformly over the surface by using an electrode behind said surface whereby the light image and corona field exist simultaneously and on the same area of the said surface,

d. simultaneously terminating both the corona discharge and the light image.

e. fixing the said image so produced to said surface while under the influence of said corona discharge and light image. and

f. washing toner medium from the said surface where not firmly held to the said surface while maintaining dark conditions.

2. A method of producing images which comprise the steps of:

a. moving a surface on which a final image is to be produced according to a light image.

b. applying a film of photoconductive developer material through a means for wetting a surface on said surface during exposure which developer comprises a carrier liquid with an electrical resistivity in excess of IO ohm centimeter and a toner medium which receives a charge from a corona but which charge is modified by the light where the said toner medium is subjected to light whereby the electrical resistivity ofthe said toner medium varies in light and dark areas and thus varies in retention delay characteristic on said surface in said field,

c. exposing said surface on which a final image is to be produced to a light image,

d. simultaneously directing a corona from an electrode remote of the said surface to the same said surface to extend uniformly over the surface by using an electrode behind said surface whereby the light image and corona field exist simultaneously and on the same area of said surface.

e. fixing said image so produced to said surface while under the influence of said corona discharge and light image. simultaneously terminating both the corona dis charge and the light image,

g. and removing said toner medium from said surface through a means for washing said surface so that said toner medium not fixed to said surface is removed.

3. The method of claim I wherein the said surface. such as a sheet of plain paper. is first moved to the exposure position through a developer applicator station, is stopped for simultaneous charging of the developer medium and light exposure. and under dark conditions is then moved through a washing station.

4. The method of claim 1 wherein the said surface travels continuously through a series of stations. the

first being a developer application station where the surface is wetted with developer. the second station being the corona charging station for the developer and the simultaneous light exposure through a slit associated with a traversing scanner whereby the image is progressively formed on the said surface as it passes the said slit. the third station being a washing and drying station through which the said surface passes.

S. The method of claim 1 wherein the said surface travels continuously through a series of stations. the first being a developer application station where the surface is wetted with developer by a roller which carries the developer to the said surface. the second station being the corona charging station for the developer and the simultaneous light exposure through a slit associated with a traversing scanner whereby the image is progressively formed on the said surface as it passes the said slit. the third station being a washing station through which the said surface passes. said washing station comprising ducts to cause liquid flow over the surface to be washed by supplying liquid to the said surface and withdrawing the liquid by suction at a locality remote from the supply locality.

6. The method of claim 1 wherein the toner particles consist of light sensitive particles the conductivity of which changes in the simultaneous presence of light and a corona discharge. said toner particles being disposed in a carrier liquid having an electrical resistivity in excess of l0 ohm. centimeter.

7. The method of claim 6 wherein the toner particles are sensitive to light of a particular colour and relatively insensitive to light of other colours. whereby a colour selection for deposition of toner can be effected from a developer having toner medium sensitive to different colour values.

8. The method of claim 1 wherein the light sensitive developer medium is a particulate photoconductor or photoinsulator suspended in a hydrocarbon solvent with an electrical resistivity in excess of 10 ohm centimeter. the photoconductor being milled in the said solvent to produce a fine wetted suspension of the photoconductor in the said solvent.

9. The method of claim 8 wherein the said developer medium contains a hydrocarbon resin or a rubbery polymer polarity regulating and fixing agent which adheres to the said photoconductor or photoinsulator and is wetted by the said solvent.

10. The method of claim 8 wherein the particulate photoconductor has its photoresponse modified by dyeing the photoconductor to increase its sensitivity to light of a selected wavelength.

11. The method of claim 8 wherein the particulate photoconductor is zinc oxide having its photoresponse modified by dyeing the zinc oxide with dyes selected from the following grouping: quinoline yellow. rose benzal. bromophenol blue. from a methyl celusolve so lution or acridine yellow. methylene blue. bromophenol blue or naphthol green from a methanol solution.

12. The method ofclaim 1 wherein the light sensitive developer medium is a coloured substance dispersed in a carrier liquid with an electrical sensitivity in excess of 10 ohm centimeter, the said coloured medium being taken from the following grouping:

Phtholocyanine blue. Benzidine yellow. Para red.

Monolite red, Toluidine red. Graphto] rubine. Cadmium red. red oxide. yellow oxide. lrgalite blue.

a light image and to a uniform corona simultaneously with the light image.

14. The method of claim l3 wherein the toner particles are in groups each responsive to conductivity changes at a different light colour and projecting on to the surface a multicoloured light imagev 

1. THE METHOD OF PRODUCING IMAGES ON A SURFACE WHICH COMPRISES: A. WETTING AND MAINTAINING A FILM OF PHOTOCONDUCTIVE DEVELOPER MATERIAL ON SAID SURFACE, WHICH DEVELOPR COMPRISES A CARRIER LIQUID WITH AN ELECTRICAL RESISTIVITY IN EXCESS OF 10**6 OHM CENTIMETER AND A TONER MEDIUM WHICH CAN RECEIVE A CHARGE FROM A CORONA BUT WHICH CHARGE IS MODIFIED BY LIGHT WHEN AND WHERE THE SAID TONER MEDIUM IS SUBJECTED TO LIGHT WHEREBY THE ELECTRICAL RESISTIVITY OF THE SAID TONER MEDIUM VARIES IN LIGHT AND DARK AREAS AND THUS VARIES IN RETENTION DELAY CHARACTERISTICS ON THE SAID SURFACE WHEN IN A FIELD, B. EXPOSING THE SAID SURFACE WITH THE WET FILM OF PHOTOCONDUCTIVE TONER THEREON TO A LIGHT IMAGE, C. SIMULTANEOUSLY DIRECTING A CORONA FROM AN ELECTRODE REMOTE OF THE SAID SURFACE TO THE SAID SURFACE TO EXTEND UNIFORMLY OVER THE SURFACE BY USING AN ELECTRODE BEHIND SAID SURFACE WHEREBY THE LIGHT IMAGE AND CORONA FIELD EXIST SIMULTANEOUSLY AND ON THE SAME AREA OF THE SAID SURFACE, D. SIMULTANEOUSLY TERMINATING BOTH THE CORONA DISCHARGE AND THE LIGHT IMAGE, E. FIXING THE SAID IMAGE SO PRODUCED TO SAID SURFACE WHILE UNDER THE INFLUENCE OF SAID CORONA DISCHARGE AND LIGHT IMAGE, AND F. WASHING TONER MEDIUM FROM THE SAID SURFACE WHERE NOT FIRMLY HELD TO THE SAID SURFACE WHILE MAINTAINING DARK CONDITIONS.
 2. A method of producing images which comprise the steps of: a. moving a surface on which a final image is to be produced according to a light image, b. applying a film of photoconductive developer material through a means for wetting a surface on said surface during exposure which developer comprises a carrier liquid with an electrical resistivity in excess of 106 ohm centimeter and a toner medium which receives a charge from a corona but which charge is modified by the light where the said toner medium is subjected to light whereby the electrical resistivity of the said toner medium varies in light and dark areas and thus varies in retention delay characteristic on said surface in said field, c. exposing said surface on which a final image is to be produced to a light image, d. simultaneously directing a corona from an electrode remote of the said surface to the same said surface to extend uniformly over the surface by using an electrode behind said surface whereby the light image and corona field exist simultaneously and on the same area of said surface, e. fixing said image so produced to said surface while under the influence of said corona discharge and light image, f. simultaneously terminating both the corona discharge and the light image, g. and removing said toner medium from said surface through a means for washing said surface so that saiD toner medium not fixed to said surface is removed.
 3. The method of claim 1 wherein the said surface, such as a sheet of plain paper, is first moved to the exposure position through a developer applicator station, is stopped for simultaneous charging of the developer medium and light exposure, and under dark conditions is then moved through a washing station.
 4. The method of claim 1 wherein the said surface travels continuously through a series of stations, the first being a developer application station where the surface is wetted with developer, the second station being the corona charging station for the developer and the simultaneous light exposure through a slit associated with a traversing scanner whereby the image is progressively formed on the said surface as it passes the said slit, the third station being a washing and drying station through which the said surface passes.
 5. The method of claim 1 wherein the said surface travels continuously through a series of stations, the first being a developer application station where the surface is wetted with developer by a roller which carries the developer to the said surface, the second station being the corona charging station for the developer and the simultaneous light exposure through a slit associated with a traversing scanner whereby the image is progressively formed on the said surface as it passes the said slit, the third station being a washing station through which the said surface passes, said washing station comprising ducts to cause liquid flow over the surface to be washed by supplying liquid to the said surface and withdrawing the liquid by suction at a locality remote from the supply locality.
 6. The method of claim 1 wherein the toner particles consist of light sensitive particles the conductivity of which changes in the simultaneous presence of light and a corona discharge, said toner particles being disposed in a carrier liquid having an electrical resistivity in excess of 106 ohm. centimeter.
 7. The method of claim 6 wherein the toner particles are sensitive to light of a particular colour and relatively insensitive to light of other colours, whereby a colour selection for deposition of toner can be effected from a developer having toner medium sensitive to different colour values.
 8. The method of claim 1 wherein the light sensitive developer medium is a particulate photoconductor or photoinsulator suspended in a hydrocarbon solvent with an electrical resistivity in excess of 106 ohm centimeter, the photoconductor being milled in the said solvent to produce a fine wetted suspension of the photoconductor in the said solvent.
 9. The method of claim 8 wherein the said developer medium contains a hydrocarbon resin or a rubbery polymer polarity regulating and fixing agent which adheres to the said photoconductor or photoinsulator and is wetted by the said solvent.
 10. The method of claim 8 wherein the particulate photoconductor has its photoresponse modified by dyeing the photoconductor to increase its sensitivity to light of a selected wavelength.
 11. The method of claim 8 wherein the particulate photoconductor is zinc oxide having its photoresponse modified by dyeing the zinc oxide with dyes selected from the following grouping: quinoline yellow, rose benzal, bromophenol blue, from a methyl celusolve solution or acridine yellow, methylene blue, bromophenol blue or naphthol green from a methanol solution.
 12. The method of claim 1 wherein the light sensitive developer medium is a coloured substance dispersed in a carrier liquid with an electrical sensitivity in excess of 106 ohm centimeter, the said coloured medium being taken from the following grouping: Phtholocyanine blue, Benzidine yellow, Para red, Monolite red, Toluidine red, Graphtol rubine, Cadmium red, red oxide, yellow oxide, Irgalite blue, Monoazo blue, Monoazo yellow and Ultramarine blue.
 13. The method of producing images which compriseS uniformly subjecting a surface to a liquid developer comprising toner particles suspended in an insulating but semiconductive liquid, said toner being selected to have a variable relative conductivity between dark and light condition, and subjecting such surface to a light image and to a uniform corona simultaneously with the light image.
 14. The method of claim 13 wherein the toner particles are in groups each responsive to conductivity changes at a different light colour, and projecting on to the surface a multicoloured light image. 